var html = document.getElementsByTagName('html')[0]
var w = document.documentElement.clientWidth// 当前屏幕可视宽度
html.style.fontSize = (w > 750 ? 640 / 750 * 100 : w / 750 * 100) + 'px'
window.onresize = function () {
  var w = document.documentElement.clientWidth
  html.style.fontSize = (w > 750 ? 640 / 750 * 100 : w / 750 * 100) + 'px'
}
